Early studies of lanthanide reagents simply sought to develop reactivity patterns mimicking those of more established organolithium or Grignard reagents. However, as the complexity of organic molecules requiring synthesis increased, demands for more highly selective reagents heightened accordingly. Consequently, the search for reagents which would complement those of the traditional organometallic nucleophiles brought more serious attention to the lanthanides, and an explosive expansion in the application of lanthanide reagents to organic synthesis began.
The perception that lanthanide metals are rare and therefore inaccessible or expensive may be a contributing factor to their long-lasting neglect and slow development as useful synthetic tools. In fact, “rare earths” in general are relatively plentiful in terms of their abundance in the earth's crust. Modem separation methods have made virtually all of the lanthanides readily available in pure form at reasonable cost.
It may be the special combination of inherent physical and chemical properties of the lanthanides that set them apart from all other elements, and provides a unique niche for these elements and their derivatives in selective organic synthesis. The lanthanides as a group are quite electropositive, for example electronegativities of samarium and ytterbium are 1.07 and 1.06, respectively, on the Allred-Rochow scale, and the chemistry of these elements is predominantly ionic. This may be because the 4f-electrons do not have significant radial extension beyond the filled 5s25p6 orbitals of the xenon inert gas core. The lanthanides therefore behave as close-shell inert gasses with a tripositive charge, and in general electrostatic and steric interactions play a greater role in chemistry of the lanthanides than do interactions between the metal and associated ligand orbitals.
Compared with transition metals, the ionic radii of the lanthanides are large. Most transition metal ionic radii lie in the range from 0.6 to 1.0 Å, whereas the lanthanides have an average ionic radius of approximately 1.2 Å. Divalent species are, of course, even larger, eight-coordinate SmII has an ionic radius of 1.41 Å, for example. The relatively large ionic radii of the lanthanides allow the accommodation of up to 12 ligands in the coordination sphere, and coordination numbers of seven, eight and nine are common. Owing to the well known lanthanide contraction, ionic radii decrease steadily across the row of lanthanides in the periodic table. The lanthanide contraction may be a consequence of poor shielding of the 4f-electrons, resulting in an increase effective nuclear charge and a concomitant decrease in ionic radius. As expected, higher coordination numbers are most common in the larger, early lanthanides.
According to the concept of hard and soft acids and bases (HSAB) established by Pearson, lanthanide +3 ions are considered to be hard acids, falling between Mg(II) and Ti(IV) in the established scale. Lanthanides therefore complex preferentially to hard bases such as oxygen donor ligands.
The strong affinity of lanthanides for oxygen is further evidenced by the bond dissociation energies for the gas phase dissociation of diatomic lanthanide oxides (LnO). For example, although they are among the lowest values for lanthanides, both SmO (136 kcal/mol; 1 cal=4.18J) and Yb (95 kcal/mol) exhibit values significantly higher than that for MgO (86 kcal/mol). This demonstrates oxophilicity (strong metal-oxygen bonds and hard Lewis acid character) has been used to great advantage in organic synthesis. These properties have been exploited extensively to enhance carbonyl substrate reactivity, and also control stereochemistry in carbonyl addition reactions through chelation. Of particular significance is the exploitation of the lanthanide(II). To date, this important divalent state involves only three metals: Eu(II), Yb(II) and Sm(II).
Although the use of Eu(II), Yb(II) and Sm(II) is quite popular, there exist many drawbacks. For example, samarium diiodide, or its complex SmI2 (tretrahydrofuran (THF))x, has become a popular reducing agent in organic synthesis. It may be used under a variety of conditions as a reducing agent to accomplish a wide range of transformations. However, in many cases the reducing power of SmI2 or SmI2(THF)x needs to be enhanced by the addition of, for example, hexamethylphosphoramide (HMPA). Unfortunately, this most effective and popular additive may be highly carcinogenic and alternatives are highly desirable. Several other methods for increasing SmI2 reactivity have been reported and include the addition of transition metal salts or samarium metal, photolysis, and the design of intramolecular reactions. However, such methods of enhancing SmI2 reactivity remain inadequate.
There is a need for the identification, synthesis and development of new lanthanide reducing agents which are more reactive than that of Eu(II), Yb(II) and Sm(II).